The E. coli K1 capsule is involved in pathogenic processes such as virulence and invasiveness. We find that expression of K1 capsule synthesis on the bacterial cell surface requires the presence of porin proteins in the outer membrane. A specific porin, protein K, found in all K1 encapsulated strains, is able to support the more rapid expression of K1 capsule as compared to other porin proteins tested. There was, however, no increase in the steady state amount of K1 capsule in strains containing protein K. Thus, protein K may be more efficient in the organization of a putative outer membrane complex involved in the extracellular transport on the K1 capsule. Bacteria are ingested and degraded by macrophages. However, certain bacterial components such as LPS may persist for several days within the macrophage after phagacytosis. Outer membrane proteins also persist. It is notable that these proteins are associated with LPS within the cell and their persistence may well be a reflection of this association. The bacterial components resistant to degradation may subsequently be cycled to the cell surface for antigen presentation. The O-PS portion of LPS in the outer membrane of Salmonellae regulates the extent of deposition of the complement component C3b and hence regulates the rate of phagacytosis of these cells by macrophages. Relatively minor changes in the structure of different O-PS found in various strains may have profound effects on the clearance of these cells and thus alter their virulence in mice. The O-PS functions in this process by influencing the activation of complement by the alternate pathway by altering the binding of factor B. and hence the amplification of deposition of C3b.